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Non-24-hour sleep-wake disorder (Non-24) is a chronic circadian rhythm sleep disorder defined as "a chronic steady pattern comprising one- to two-hour daily delays in sleep onset and wake times in an individual living in society"he pattern of delay persists literally "around the clock", typically taking a few weeks to complete one cycle. This disruption of the body clock causes cyclical bouts of nighttime sleeplessness and excessive daytime fatigue and napping.

Non-24 is a neurological sleep disorder. It has been traced to a number of genetic abnormalities that affect specific
biologic factors in the brain. While both sighted and blind people are diagnosed with Non-24, the disorder affects proportionately more totally blind individuals than sighted.

In people with this disorder, the body essentially insists that the day is longer than 24 hours and refuses to adjust to the external light/dark cycle. This makes it impossible to sleep at normal times and also causes daily shifts in other aspects of the circadian rhythms such as peak time of alertness, body temperature minimum and hormone secretion. Left untreated, non-24-hour sleep-wake syndrome causes a person's sleep-wake cycle to change every day, the degree determined by how much over 24 hours the cycle lasts. The cycle progresses around the clock, eventually returning to "normal" for one or two days before going "off" again. This is known as free-running sleep. Most cases that have been reported in the medical literature have occurred in blind patients; Non-24 occurs in more than half of all people who are totally blind.

People with the disorder may have an especially hard time adjusting to changes in "regular" sleep-wake cycles, such as vacations, stress, evening activities, time changes like daylight saving time, travel to different time zones, illness, medications (especially stimulants or sedatives), changes in daylight hours in different seasons, and growth spurts, which are typically known to cause fatigue. They also show lower sleep propensity after total sleep deprivation.